Work machines such as wheel type loaders include work implements capable of being moved through a number of positions during a work cycle. Such implements typically include buckets, forks, and other material handling apparatus. The typical work cycle associated with a bucket includes sequentially positioning the bucket and associated lift arm in a digging position for filling the bucket with material, a carrying position, a raised position, and a dumping position for removing material from the bucket.
Control levers are mounted at the operator's station and are connected to an electrohydraulic circuit for moving the bucket and/or lift arms. The operator must manually move the control levers to open and close hydraulic valves that direct pressurized fluid to hydraulic cylinders which in turn cause the implement to move. For example, when the lift arms are to be raised, the operator moves the control lever associated with the lift arm hydraulic circuit to a position at which a hydraulic valve causes pressurized fluid to flow to the head end of a lift cylinder, thus causing the lift arms to rise. When the control lever returns to a neutral position, the hydraulic valve closes and pressurized fluid no longer flows to the lift cylinder.
In normal operation, the work implement is often abruptly started or brought to an abrupt stop after performing a desired work cycle function, which results in rapid changes in velocity and acceleration of the bucket and/or lift arm, machine, and operator. This can occur, for example, when the implement is moved to the end of its desired range of motion. The geometric relationship between the linear movement of the tilt or lift cylinders and the corresponding angular movement of the bucket or lift arm can produce operator discomfort as a result of the rapid changes in velocity and acceleration. The forces absorbed by the linkage assembly and the associated hydraulic circuitry may result in increased maintenance and accelerated failure of the associated parts. Another potential result of the geometric relationship is excessive angular rotation of the lift arm or bucket near some linear cylinder positions which may result in poor performance.
Stresses are also produced when the vehicle is lowering a load and operator quickly closes the associated hydraulic valve. The inertia of the load and implement exerts forces on the lift arm assembly and hydraulic system when the associated hydraulic valve is quickly closed and the motion of the lift arms is abruptly stopped. Such stops cause increased wear on the vehicles and reduce operator comfort. In some situations, the rear of the machine can even be raised off of the ground.
Further, prior methods and apparatus have suffered from inconsistent control of rate of motion and stopping position. This inconsistent control is believed to be a result of controlling solely on velocity or by scaling the operator command signal.
The present invention is directed to overcoming one or more of the problems as set forth above.